Conventionally, there has been known an imaging device which mounts thereon an optical filter which intentionally shifts focus except a subject, what is called adjusts a blurred image in order to highlight part of the subject in a photographed image. Specifically, an apodized filter can be cited as an example, and for example, there has been disclosed a filter which is constituted so that transmittance of light decreases as separating from an optical center in a vertical direction to an optical axis, or the like [for example, refer to Reference 1 (WO 2013-161767 A1)].
By using such a filter, in particular, it is possible to relax a contour of the blurred image in a scene of portrait photographing, macro photographing, and the like in which depth of focus is shallow and obtain a high-quality image in which an aimed subject is shown up. On the other hand, in an apodized filter having a unique characteristic, namely, a fixed (unchangeable) characteristic in a transmittance distribution, the contour relaxation of the blurred image and a diameter of the blurred image are affected and so-called blurriness is impaired depending on the degree of transmittance of a peripheral edge portion and a central portion of the filter. Further, there has been known a problem that an exposure time is subjected to constraint by light intensity shortage, or the like.
With respect to the above-described problem, in a similar manner that a specific filter is selected mechanically by mounting a plurality of ND (Neutral Density) filters on a rotating rotor and interlocking them with a blade aperture, the above-described constraint can be avoided by mechanically replacing a plurality of apodized filters having a plurality of optical specifications depending on a photographing condition. However, the imaging device is required to become miniature and thin, and when, in particular, the plurality of apodized filters are mounted on a camera such as a mobile phone and a tablet PC which has constraint on a thickness, a mechanism part becomes complicated and it is spatially very difficult to practically mount them.
Therefore, as an apodized filter by which spatial advantage and flexibility of the transmittance distribution can be obtained, (one) apodized filter in which an optional optical characteristic can be selected actively by drive is required.
Meanwhile, in a solid-state imaging device which is mounted on a digital video camera, a digital still camera, and the like, there has been proposed an optical element which adjusts brightness using a light control function of an electrically controllable electrochromic and an imaging device which is equipped with the optical element [for example, refer to Reference 2 (JP-A 2007-248604)].
In Reference 2 in particular, by applying an optional voltage to a concentrically provided transparent electrode pattern instead of an exposure controlling mechanism part which performs a position adjustment of the ND filter with respect to an aperture opening of the solid-state imaging device or replacement of the ND filter different in a light shielding property and an opening area, color development/reduction of electrochromic layers stacked uniformly on the transparent electrodes is controlled and transmitted light intensity is adjusted concentrically, and thereby an aperture function is exhibited. Thus, applying gradient electric potential concentrically makes it possible to form a pseudo continuous transmitted light intensity distribution and obtain the aperture function. However, there has been a problem that a plurality of transparent electrodes are each used as an independent transparent electrode, thereby causing a boundary of electric potential (gap between adjacent transparent electrodes), and further a plurality of routed wiring parts for power feeding are necessary toward a central portion of the concentric circles, thereby inevitably causing light intensity unevenness as an aperture.
Under such circumstances, there has been proposed an apodized filter in which in a cell sandwiched by two transparent conductive layer-attached substrates, an inner surface of one of the substrates is formed as non-flatness, and an electrochromic solution is sealed in the cell [for example, refer to Reference 3 (JP-A 2012-510649)]. Reference 3 discloses that an applied voltage from the outside causes the color development/reduction of this electrochromic solution, and thereby transmitted light intensity of light can be varied by a coloring concentration distribution derived from the non-flatness and an optical characteristic can be controlled from outside.